And on Sunday, the San Anselmo man, 56, will cross a special threshold: 100,000 miles. That's the equivalent of running around the world more than four times.

By Fanelli's estimate, he will hit that milestone about halfway through the 26.2-mile Philadelphia Marathon, which is where he began running marathons four decades ago.

Fanelli makes his money as a real estate agent. But running endless stretches of trail and track, and keeping a record of each of them, has been his lifeblood since he was a teenager.

"Some people say I'm compulsive," he said. "But I say it's more just competitive."

Growing up in Philadelphia, Fanelli jogged with his father as a child. He was glued to the television as he watched the U.S. sprinters compete at the 1968 Mexico City Olympics.

In 1970, he made it onto his high school's cross-country team and began a meticulous diary of the lengths and times of his races, which he used to gauge his progress.

S.F. State record

After he moved to the Bay Area in 1976, his running diary grew longer. The entries reflect his fastest marathon time (2 hours, 25 minutes, 1980) and his 10,000-meters race at San Francisco State University, a school record that still stands (31 minutes, 6 seconds, 1981).

The entries also show his back-to-back wins of the San Francisco Pacific Rim Marathon in 1988 and 1989, and the 100-mile Santa Rosa race he finished in 16 hours and 42 minutes.

There's no way to know how many athletes have racked up 100,000 miles, but few serious runners have achieved even half that mileage over the same amount of time, said Dr. Anthony Luke, UCSF's director of primary care sports medicine and medical director of the San Francisco Marathon.

"I can do a lot of my very best thinking when I'm running," he said. "I'm creating solutions to difficult transactions I'm working on. It's a great way to start the day."

As a member of the Olympic Club, the nation's oldest athletic club, and the Tamalpa Runners, a Marin running club, Fanelli does longer runs through the streets of San Francisco and on the tracks at San Francisco State University and the College of Marin. In an average week, he logs 50 miles, he said.

Slowing with age

Fanelli said his body can no longer handle the pace of his younger years. Back then, he ran 100 miles a week. Six years ago, he could run a mile in less than 5 minutes, but while he remains quick for his age, he has stopped trying to be the fastest.

Instead, he relishes the satisfaction of conquering long distances and started contemplating the 100,000-mile mark long ago.

"All I have to do," he recalled thinking, "is keep this up for another 16 or 17 or 20 years."

Numbers fascinate Fanelli. "He is like a walking encyclopedia of all of our times," said friend Tim Fitzpatrick, 52, of Larkspur. Fanelli has both coached him and teamed up with him to co-coach the Impala Racing Team, a local women's running club.

Whenever someone is trying to remember his or her marathon time from a decade ago, Fanelli is the person to call, Fitzpatrick said. What's more, he added, Fanelli's circle of runners considers him a role model who "makes the process of getting fit fun."

Hitting the six-digit benchmark doesn't come without some bodily damage. Fanelli said his knees are fine after years of impact, but his feet aren't. In particular, the pad on the ball of his left foot has rubbed away, "so it's pretty much just bone on surface," he said.

Fanelli plans to undergo surgery after the Philadelphia Marathon.

Running is associated with plenty of health benefits, such as lower rates of heart disease, diabetes, cancer and aging-associated disability. But recent studies have suggested that extreme distance running - like a lifetime of marathons - may cancel out some of the benefits of exercise.

Continuous distance running isn't for everyone, said UCSF's Luke.

"For some people, I think getting rest in there is going to be more advantageous" in terms of achieving running goals, he said.

Fiercely competitive

Fanelli's wife - a runner who met him at the 1996 Boston Marathon - isn't surprised that painful feet haven't slowed her husband down. He once nearly missed a flight from London, she said, because he was doing his daily run outside the airport.

"I think that he's fiercely competitive," said Renay Weissberger Fanelli, "and so when he sets his mind to something, he's going to accomplish it come hell or high water."

Mike Fanelli is already thinking ahead to his next big race, the 2013 Summer National Senior Games in Cleveland, for which he has qualified in the 800- and 1,500-meter races.

But first, he'll pause at the finish line of the Philadelphia Marathon to savor the moment.

"I'm in my final 100 miles," he said. "I can see the light at the end of the tunnel."